Each participant filled the survey for multiple times, and the total number of times the survey was delivered is 4. Participant who did not reply at one of the four times will not have any record in the dataset. For example, 001 have all 4 inputs from 1-4, so didn't have any missing, but 002 missed 1, 4; 003 missed 1, 2, 3; 004 missed 2. How can I insert rows for those missing participants so that each id has 4 rows in the dataset like this: